Using information from regional weather reports during the time the surveys were collected, they found that those respondents who reported sleeping badly were most affected in warm weather, with particularly hot summers causing spikes in rates of insomnia.

Using a statistical model, they found that for every for every month with an average temperature 1 degree Celsius (32 degrees Fahrenheit) higher than normal, three out of every 100 people lost about a night of sleep.

The study identified that those most affected by sleeplessness were elderly and low-income respondents. Those earning less than $50,000 on average experienced three times as many nights of disrupted sleep. Study authors linked this pattern to their inability to afford an air conditioner.

They argue that if global warming isn’t slowed by the end of the century, it could lead to millions of Americans experiencing disrupted sleep—which could impact health in a number of ways.

“Too little sleep can make a person more susceptible to disease and chronic illness, and it can harm psychological well-being and cognitive functioning,” Obradovich said in a press release.